In 1915, Albert Einstein proposed the General Theory of Relativity (GR), in which gravity is described by the distortion of the fabric of spacetime surrounding massive cosmic objects. Einstein’s theory also predicted that perturbations in this web of spacetime would produce gravitational waves (GW) that carry information about their sources. Collisions of massive cosmic objects, such as black holes and/or neutron stars, can produce such GWs. Detections of such merging binaries by a global array of GW observatories, such as the advanced LIGO/Virgo and KAGRA (LVK) detector network, can therefore provide a unique opportunity to test the predictions of GR and also search for any yet unknown physics beyond GR. Gravitational waves are ideal messengers to test whether Einstein’s GR theory is right!